1. Field of the Invention
The invention relates generally to methods for establishing a north-oriented or south-oriented line by circumpolar orientation for use in the positioning and aiming of artillery or for other uses wherein an instrument is aimed or aligned relative to a global pole by reference to circumpolar stars. In particular, the invention provides a reticle plate and methods useful therewith which can be directly applied to known optical instruments such as artillery aiming circles.
2. Description of the Prior Art
Reticles and devices employing reticles are well known in a great number of use environments wherein an instrument is aimed through alignment of the reticle with a target. Reticles may be as simple as the "cross-hairs" of a firearm or as complex as scanning rotary reticles used to facilitate guidance of spacecraft and weaponry known popularly as "guided missiles". In the patent literature, Herter et al in U.S. Pat. 3,392,450; MacMillan in U.S. Pat. 3,383,987; O'Brien in U.S. Pat. 3,190,003; and Sullivan in U.S. Pat. 2,806,287 disclose reticles which assist in range finding and aiming of various devices. Herter et al disclose target spanning devices which take the form of circles used on a reticle disc. As a second particular example, Sullivan provides a plurality of concentric ring elements of differing diameters which are used to assist the user of a device in locating the exact center of a target.
A reticle is seen to be no more in its most basic embodiment than a set of two or more fine wires or lines placed at the principle focus of a telescope lens, the reticle being in the principle focus of the eyepiece of the lens so that direction of the telescope onto an object causes an image of the object and the reticle to be in clear view within the eyepiece. Reticles in a use environment particularly directed to that of the present invention are utilized in optical instruments such as a theodolite which constitutes a sighting telescope mounted for free rotation about horizontal and vertical axes. Graduated scales on such an instrument allow measurement of angles of rotation, such measurement allowing the use of a theodolite or similar device to be used for a variety of alignment functions as well as for length measurements, the establishment of incremental spacing of divided circles, such as for tooth spacing of a large gear, and for motion observation among other things. Relatively low-accuracy theodolites have also been employed for the positioning and aiming of artillery, such optical devices being referred to as aiming circles which comprise telescopes fitted with vertical and horizontal angle measurement circles. Orientation of an instrument using such optical apparatus involves the practice of methods such as the Polaris-Kochab method, the star gazer method and the Polaris II method, as examples. The Polaris-Kochab method requires multiple sightings with recordation of readings followed by reference to conversion tables and to a map sheet. A user of the Polaris-Kochab method must be extensively trained in order to master the complexities of the method. The star gazer and Polaris II methods require that the sighting instrument be fitted with a rotating reticle, thereby necessitating a redesign of presently available optical aiming instruments. Disposition of a rotary bearing in the optical path of such an instrument with any looseness or misalignment of the bearing unavoidably degrades instrument pointing accuracy.
The present invention finds advantage over the prior art by virtue of the simplicity of the disclosed reticle plate and of the methods of the invention, practice of the invention requiring no calculation on the part of a user, no modification to the aiming circle instrument, and the reduced requirement for training of a user of the instrument. Further, the present invention can be practiced without the requirement for ancillary materials such as "look-up" tables or maps which require plotting of converted values thereon. The invention thus allows ready establishment of a north-oriented or a south-oriented line under at least partially clear night conditions by orientation relative to circumpolar stars.